Jupiter’s non-auroral ionosphere

Abstract:   An international team investigating Jupiter’s non-auroral upper atmosphere. Recent measurements of Jupiter’s equatorial regions have revealed surprising complexity in the ionosphere and thermosphere, suggesting a wide range of energy inputs and ionisation processes. This Team will draw together a combination of various current datasets of observations from the Earth and ongoing Juno measurements around Jupiter, comparing and contrasting the ionosphere with the surrounding magnetospheric environment. It will also investigate the development of ionospheremagnetosphere coupled models that reveal the drivers of these ionospheric variations.   Short-form report:   In our first meeting, we outlined the problem – Jupiter’s non-auroral ionosphere is complex and contained what was an array of unexplained features.  Firstly, the altitudinal electron densities, measured using both in-situ spacecraft measurements and radio occultations, have consistently shown highly complex morphologies, with both small-scale and larger bulk changes in ionospheric density, sometimes at much lower and higher altitudes than the predicted peak (Coffin et al. 2025, https://doi.org/10.1029/2025JA033754; Tiranti et al. 2025, https://doi.org/10.1029/2025JA034066). Secondly, across the disk, images of the ionospheric brightness revealed complex regions of brighter and darker emission. Only one of these features had a clear explanation, the dark magnetic equator was thought to be an equatorial ionospheric fountain (or perhaps ‘gutter’) where neutrals winds move across, i.e. perpendicular to, magnetic field lines, forcing ions to rise into the surrounding magnetosphere (or sink into the underlying stratosphere). Following our first meeting, we undertook a significant body of research, both with analysis of observations and modeling of the ionosphere. Roberts et al. 2025a (https://doi.org/10.3847/PSJ/adc09b) compared the first high resolution maps of upper-atmospheric temperatures to understand the timescales of temperature variability. Knowles et al., 2025a (https://doi.org/10.1029/2025JA033868) revealed how closely correlated many of the ionospheric features are to the magnetic conditions. Roberts et al., 2025b (submitted) revealed very clearly that the ionosphere is very stable in temperature and these brightness differences are almost entirely due to column density changes (i.e. changes in the ion production or destruction). Agiwal et al., 2025 (https://doi.org/10.3847/PSJ/add108) took recent modeling of Jupiter’s thermosphere (Mueller-Wodarg, 2025) and combined it with the Juno-constrained magnetic field to show that the vertical structure of Jupiter’s ionosphere is primarily governed by neutral winds and magnetic field geometry, reducing the electron density at the H3+ ionospheric peak, resulting in two essential effects.  Firstly, the electron density peak moves to higher or lower altitudes, producing changes that match with altitudinal profiles. Secondly, in regions where this happens, the H3+ destruction rate is reduced, enhancing density and thus brighter. In regions where this vertical motion is minimised, the ionosphere is left unaffected (the altitudinally-generalised ionosphere, AGI), and so appear in the maps as regions of darkness. This effect dominates most of our original H3+ mapped intensities, but one dark region exists away from the regions of predicted AGI, ‘Region B: the Northern Ionospheric Anomaly).  However, nightside UV emissions have shown this region has significant enhanced UV brightness, perhaps indicating active ion precipitation.  Though the detailed measurements have not yet been published, one measurement is included in Kurth et al., 2025 (https://doi.org/10.1029/2024JE008845), aligning directly with this region B dark region. Modeling of the radiation belt drift (presented by Domenique Freund at the second meeting) highlights that this region maps to a strange region of the magnetosphere ~1.25-1.75 RJ, that is thought to be empty.  And so, while this last mysterious darkening has a clear origin with precipitating magnetospheric material, the source of those particles remains one final mystery as yet unsolved by our team. In addition to this, a wide range of other detailed studies and comparisons were undertaken, leading to a rich range of discussion about the ionosphere of Jupiter and it’s importance in the wider solar system context.